Small molecules are important targets with the potential of clinical or commercial applications such as medical diagnostics, environmental monitoring, and forensic science. Thus, efforts to develop methods for portable, low-cost, and quantitative on-site detection of a broad range of small molecules are gaining momentum.
Cocaine is a central nervous system stimulant that increases levels of dopamine and potently inhibits neurotransmitter reuptake at the synapse. Abuse of cocaine has been shown to cause anxiety, paranoia, mood disturbances, organ damage, and violent behavior.
Synthetic cathinones (also known as bath salts) are designer drugs sharing a similar core structure with amphetamines and 3,4-methylenedioxy-methamphetamine (MDMA). They are highly addictive central nervous system stimulants, and are associated with many negative health consequences, including even death. Although these drugs have emerged only recently, abuse of bath salts has become a threat to public health and safety due to their severe toxicity, increasingly broad availability, and difficulty of regulation. More importantly, there is currently no reliable presumptive test for any synthetic cathinone. Chemical spot tests used to detect conventional drugs such as cocaine, methamphetamine, and opioids show no cross-reactivity to synthetic cathinones.
Methods that are highly sensitive and selective, including high-performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS), have been used for the detection of small molecules. However, these methods are time-consuming and require expensive reagents, advanced equipment, complex sample preparation, and/or trained operators.
Various immunoassays have also been developed for the detection of small molecules such as cocaine and/or its major metabolite benzoylecgonine in biofluids, including the enzyme-linked immunosorbent assay (ELISA). Unfortunately, the use of immunoassays for the detection of designer drugs is often limited because of the high cost of generating new antibodies and issues with narrow target binding-spectrum and poor specificity.
Nucleic acid-based bioaffinity elements, known as aptamers, can be isolated in vitro through systematic evolution of ligands by exponential enrichment (SELEX) processes to bind various targets with high specificity and affinity, including proteins, metal ions, small molecules, and even whole cells. They have gained considerable attention as bio-recognition elements with diverse applications in areas such as drug screening, medical diagnostics, and environmental monitoring. This is in part because aptamers are chemically stable, offering long shelf lives, and can be synthesized at a low cost with high reproducibility. Also, aptamers can be engineered to have tunable target-binding affinities or various functionalities. These advantages make aptamers ideal for use in biosensors.
Among the numerous aptamer-based sensing platforms, colorimetric assays are especially desirable for on-site detection, as they can be interpreted by the naked eye and do not require any specialized equipment to obtain a readout. For example, gold nanoparticles (AuNPs) have been widely employed with aptamers as sensitive colorimetric signal reporters for naked-eye small-molecule detection. G-quadruplex-structured DNA enzymes (DNAzymes) are alternative signal reporters for colorimetric aptamer-based assays. However, most of these assays offer only limited capabilities for naked-eye detection, because the resulting absorbance changes can only be detected by instruments.
Therefore, there is a need for methods and materials for rapid, naked-eye small-molecule detection. Assays using such materials and methods provide essential features such as ease of use, cost-effectiveness, rapid turnaround time and superior sensitivity and specificity.